The segment of facial nerve most commonly involved in CSOM/ cholesteatoma?

Ans. a (Tympanic part). (Ref. Radiology of ent by Valvassori 1st ed., 70)Cholesteatomas may erode the facial nerve canal and expose the nerve rarely. The most common site of facial nerve canal erosion is area extending from oval window to the pyramidal eminence and the proximal portion of the vertical segment. Erosion of the anterior part of tympanic segment and geniculate ganglion may occur in large anterior epitympanic lesions. Erosion of mastoid segment occurs in lesions which involve the entire mastoid.